Testicular cancer is the most frequent solid tumor in young men, with incidence rates for the 20-30 year old group reaching 11 per 100,000 in high incidence regions such as the San Francisco Bay Area. Recent evidence, including one case-control study, has linked certain testicular cancers with prenatal exposure to diethylstilbestrol (DES), a synthetic hormone given to several million women in the 1940's, 50's and 60's to prevent abortion. The purpose of the present proposal is to carry out a case-control study of testicular cancer and prenatal DES exposure in young men in Northern California. Cases for the proposed study will be drawn from institutions participating in the Northern California Oncology Group, including Stanford University Hospital, the University of California at San Francisco, and Children's Hospital of San Francisco. It is proposed to interview 250 cases over a three-year period. Men participating in the study will be asked to provide access to their mothers and also to peer controls who share the cases' early life experiences. The mothers of both cases and controls will be interviewed to determine their exposure to DES during pregnancy. Positive reported exposure to DES will be verified by inspection of medical records for both case and control mothers. Negative exposure to DES will be verified in a twenty percent sample of mothers. The study will be carried out by staff of the Northern California Cancer Program, the parent organization of the Northern California Oncology Group. Statistical consultation is available through the statistical consultants of the Northern California Oncology Group.